Foraminifera have been utilized to interpret the response of waters in the Gulf of St. Lawrence to climatic changes during the Holocene. Sediment cores (up to 1000 cm long) from the Gulf of St. Lawrence and environs, are characterized throughout by meager foraminiferal faunas. The microfaunas are characteristic of marginal marine environments and are typified by low foraminiferal numbers (less than 300 and generally less than 100 per unit sample), few genera and species, and hyposaline, shallow-water assemblages. The microfaunal information indicates that marine waters were more brackish and much shallower during the latter stages, and immediately following the Wisconsin glaciation. Shallow-water foraminiferal species such as Elphidium incertum clavatum, Islandiella islandica, and I. teretis are commonly the first to inhabit cool temperate to northern environments after glacial retreat. Consequently, because of the absence of deep water marine microfaunas it is believed that many areas in the Gulf of St. Lawrence were at least 100 to 200 m shallower than at present.The Holocene history of the area is one of transition from a rapidly fluctuating brackish water environment, to one which is more consistent with the present environment. The presence of Globigerinoides ruber (pink), a species commonly associated with subtropical waters, intermixed with the eurybathic benthonic fauna, indicates distinctive lateral and vertical water-mass zonation in a restricted geographic area. Warm water incursions into the Gulf of St. Lawrence from the Gulf Stream, which contained the subtropical foraminiferal species Globigerinoides ruber and Globorotalia menardii, were intermittent, whereas a persistent cold-water marine influence from the Arctic via the Labrador Current is indicated by the presence of Globigerina pachyderma. The adjoining Scotian Shelf faunas, alternating from sparse, to prolific and diverse, during the Holocene, suggest that conditions there were not significantly different from those in the Gulf of St. Lawrence.Extremely brackish and/or shallower waters were present in most of the western Laurentian Channel and shelf waters of the Gulf of St. Lawrence until recently (< 6400 ± 130 yr B.P.). The baymouth bars, persistent features restricting circulation with most bays, estuaries, and lagoons adjoining Prince Edward Island and New Brunswick were established between 4540 ± 180 and 2235 ± 155 yr B.P. Sediments containing subtropic and arctic planktonic species, alternating with eurythermal benthonic species, are indicative of environmental extremes throughout the Holocene. It is believed that many of these marine fluctuations were neither recorded nor preserved in adjoining continental sediments of equivalent age.